Potassium ferrocyanide, also known as yellow prussiate of potash or potassium hexacyanidoferrate(II), is a coordination compound of formula K4[Fe(CN)6]•3H2O, which forms lemon-yellow monoclinic crystals at room temperature, and which decomposes at its boiling point.
Potassium ferrocyanide is the product of the reaction between hydrogen ferrocyanide and potassium hydroxide:
The reaction forms a stable compound that is neither combustible nor pyrophoric. This compound is a strong reducing agent and is thus incompatible with oxidizing agents. Addition of metal chlorates, perchlorates, nitrates, or nitrites to a solution of carefully prepared and otherwise stable potassium ferrocyanide may result in a large explosion.
Although it is insoluble in alcohol, a liter of water can dissolve just under three hundred grams of the crystals, and the solution can react with acid to release toxic hydrogen cyanide (HCN) gas. The resulting HCN gas boils at 26 °C and, being lighter than air (with a gaseous density of 0.94), quickly evaporates clear of the release point.
When chlorine gas is bubbled through a solution of this compound, potassium ferricyanide is formed in the reaction:
This reaction can be used to remove potassium ferrocyanide from a solution. When the two are combined, the product is Prussian blue. Potassium ferrocyanide, potassium ferricyanide, and Prussian blue account for over 97% of cyanides in the environment.
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